Chapter 1 Why should you learn to write
programs?
Writing programs (or programming) is a very creative and rewarding activity. You can write
programs for many reasons ranging from making your living to solving a difficult data analysis
problem to having fun to helping someone else solve a problem. This book assumes that
everyone needs to know how to program and that once you know how to program, you will
figure out what you want to do with your newfound skills.
We are surrounded in our daily lives with computers ranging from laptops to cell phones. We
can think of these computers as our "personal assistants" who can take care of many things on
our behalf. The hardware in our current-day computers is essentially built to continuously ask us
the question, "What would you like me to do next?".
Programmers add an operating system and a set of applications to the hardware and we end up
with a Personal Digital Assistant that is quite helpful and capable of helping many different
things.
Our computers are fast and have vast amounts of memory and could be very helpful to us if we
only knew the language to speak to explain to the computer what we would like it to "do next". If
we knew this language we could tell the computer to do tasks on our behalf that were repetitive.
Interestingly, the kinds of things computers can do best are often the kinds of things that we
humans find boring and mind-numbing.
For example, look at the first three paragraphs of this chapter and tell me the most commonly
used word and how many times the word is used. While you were able to read and understand
the words in a few seconds, counting them is almost painful because it is not the kind of problem
that human minds are designed to solve. For a computer the opposite is true, reading and
understanding text from a piece of paper is hard for a computer to do but counting the words and
telling you how many times the most used word was used is very easy for the computer:
python words.py
Enter file:words.txt
to 16
Our "personal information analysis assistant" quickly told us that the word "to" was used sixteen
times in the first three paragraphs of this chapter.
This very fact that computers are good at things that humans are not is why you need to become
skilled at talking "computer language". Once you learn this new language, you can delegate
mundane tasks to your partner (the computer), leaving more time for you to do the things that
you are uniquely suited for. You bring creativity, intuition, and inventiveness to this partnership.
1.1 Creativity and motivation
While this book is not intended for professional programmers, professional programming can be
a very rewarding job both financially and personally. Building useful, elegant, and clever
programs for others to use is a very creative activity. Your computer or Personal Digital
Assistant (PDA) usually contains many different programs from many different groups of
programmers, each competing for your attention and interest. They try their best to meet your
needs and give you a great user experience in the process. In some situations, when you choose a
piece of software, the programmers are directly compensated because of your choice.
If we think of programs as the creative output of groups of programmers, perhaps the following
figure is a more sensible version of our PDA:
For now, our primary motivation is not to make money or please end-users, but instead for us to
be more productive in handling the data and information that we will encounter in our lives.
When you first start, you will be both the programmer and end-user of your programs. As you
gain skill as a programmer and programming feels more creative to you, your thoughts may turn
toward developing programs for others.
1.2 Computer hardware architecture
Before we start learning the language we speak to give instructions to computers to develop
software, we need to learn a small amount about how computers are built. If you were to take
apart your computer or cell phone and look deep inside, you would find the following parts:
The high-level definitions of these parts are as follows:

The Central Processing Unit (or CPU) is that part of the computer that is built to be
obsessed with "what is next?". If your computer is rated at 3.0 Gigahertz, it means that
the CPU will ask "What next?" three billion times per second. You are going to have to
learn how to talk fast to keep up with the CPU.

The Main Memory is used to store information that the CPU needs in a hurry. The main
memory is nearly as fast as the CPU. But the information stored in the main memory
vanishes when the computer is turned off.

The Secondary Memory is also used to store information, but it is much slower than the
main memory. The advantage of the secondary memory is that it can store information
even when there is no power to the computer. Examples of secondary memory are disk
drives or flash memory (typically found in USB sticks and portable music players).

The Input and Output Devices are simply our screen, keyboard, mouse, microphone,
speaker, touchpad, etc. They are all of the ways we interact with the computer.

These days, most computers also have a Network Connection to retrieve information
over a network. We can think of the network as a very slow place to store and retrieve
data that might not always be "up". So in a sense, the network is a slower and at times
unreliable form of Secondary Memory
While most of the detail of how these components work is best left to computer builders, it helps
to have some terminology so we can talk about these different parts as we write our programs.
As a programmer, your job is to use and orchestrate each of these resources to solve the problem
that you need solving and analyze the data you need. As a programmer you will mostly be
"talking" to the CPU and telling it what to do next. Sometimes you will tell the CPU to use the
main memory, secondary memory, network, or the input/output devices.
You need to be the person who answers the CPU's "What next?" question. But it would be very
uncomfortable to shrink you down to 5mm tall and insert you into the computer just so you could
issue a command three billion times per second. So instead, you must write down your
instructions in advance. We call these stored instructions a program and the act of writing these
instructions down and getting the instructions to be correct programming.
1.3 Understanding programming
In the rest of this book, we will try to turn you into a person who is skilled in the art of
programming. In the end you will be a programmer --- perhaps not a professional programmer
but at least you will have the skills to look at a data/information analysis problem and develop a
program to solve the problem.
In a sense, you need two skills to be a programmer:

First you need to know the programming language (Python) - you need to know the
vocabulary and the grammar. You need to be able spell the words in this new language
properly and how to construct well-formed "sentences" in this new languages.

Second you need to "tell a story". In writing a story, you combine words and sentences to
convey an idea to the reader. There is a skill and art in constructing the story and skill in
story writing is improved by doing some writing and getting some feedback. In
programming, our program is the "story" and the problem you are trying to solve is the
"idea".
Once you learn one programming language such as Python, you will find it much easier to learn
a second programming language such as JavaScript or C++. The new programming language has
very different vocabulary and grammar but once you learn problem solving skills, they will be
the same across all programming languages.
You will learn the "vocabulary" and "sentences" of Python pretty quickly. It will take longer for
you to be able to write a coherent program to solve a brand new problem. We teach
programming much like we teach writing. We start reading and explaining programs and then we
write simple programs and then write increasingly complex programs over time. At some point
you "get your muse" and see the patterns on your own and can see more naturally how to take a
problem and write a program that solves that problem. And once you get to that point,
programming becomes a very pleasant and creative process.
We start with the vocabulary and structure of Python programs. Be patient as the simple
examples remind you of when you started reading for the first time.
1.4 Words and sentences
Unlike human languages, the Python vocabulary is actually pretty small. We call this
"vocabulary" the "reserved words". These are words that have very special meaning to Python.
When Python sees these words in a Python program, they have one and only one meaning to
Python. Later as you write programs you will make your own words that have meaning to you
called variables. You will have great latitude in choosing your names for your variables, but you
cannot use any of Python's reserved words as a name for a variable.
In a sense, when we train a dog, we would use special words like, "sit", "stay", and "fetch". Also
when you talk to a dog and don't use any of the reserved words, they just look at you with a
quizzical look on their faces until you say a reserved word. For example, if you say, "I wish more
people would walk to improve their overall health.", what most dogs likely hear is, "blah blah
blah walk blah blah blah blah." That is because "walk" is a reserved word in dog language.
Many might suggest that the language between humans and cats has no reserved words1.
The reserved words in the language where humans talk to Python incudes the following:
and
del
for
is
raise
assert
elif
from
lambda
return
break
else
global
not
try
class
except
if
or
while
continue
exec
import
pass
yield
def
ï¬ nally
in
print
That is it, and unlike a dog, Python is already completely trained. When you say "try", Python
will try every time you say it without fail.
We will learn these reserved words and how they are used in good time, but for now we will
focus on the Python equivalent of "speak" (in human to dog language). The nice thing about
telling Python to speak is that we can even tell it what to say by giving it a message in quotes:
print 'Hello world!'
And we have even written our first syntactically correct Python sentence. Our sentence starts
with the reserved word print followed by a string of text of our choosing enclosed in single
quotes.
1.5 Conversing with Python
Now that we have a word and a simple sentence that we know in Python, we need to know how
to start a conversation with Python to test our new language skills.
Before you can converse with Python, you must first install the Python software on your
computer and learn how to start Python on your computer. That is too much detail for this
chapter so I suggest that you consult www.pythonlearn.com where I have detailed instructions
and screencasts of setting up and starting Python on Macintosh and Windows systems. At some
point, you will be in a terminal or command window and you will type python and the Python
interpreter will start executing in interactive mode: and appear somewhat as follows:
Python 2.6.1 (r261:67515, Jun 24 2010, 21:47:49)
[GCC 4.2.1 (Apple Inc. build 5646)] on darwin
Type "help", "copyright", "credits" or "license" for more
information.
>>>
The >>> prompt is the Python interpreter's way of asking you, "What do you want me to do
next?". Python is ready to have a conversation with you. All you have to know is how to speak
the Python language and you can have a conversation.
Lets say for example that you did not know even the simplest Python language words or
sentences. You might want to use the standard line that astronauts use when they land on a far
away planet and try to speak with the inhabitants of the planet:
>>> I come in peace, please take me to your leader
File "<stdin>", line 1
I come in peace, please take me to your leader
^
SyntaxError: invalid syntax
>>>
This is not going so well. Unless you think of something quickly, the inhabitants of the planet are
likely to stab you with their spears, put you on a spit, roast you over a fire, and eat you for
dinner.
Luckily you brought a copy of this book on your travels and you thumb to this very page and try
again:
>>> print 'Hello world!'
Hello world!
This is looking much better so you try to communicate some more:
>>> print 'You must be the legendary god that comes from
the sky'
You must be the legendary god that comes from the sky
>>> print 'We have been waiting for you for a long time'
We have been waiting for you for a long time
>>> print 'Our legend says you will be very tasty with
mustard'
Our legend says you will be very tasty with mustard
>>> print 'We will have a feast tonight unless you say
File "<stdin>", line 1
print 'We will have a feast tonight unless you say
^
SyntaxError: EOL while scanning string literal
>>>
The conversation was going so well for a while and then you made the tiniest mistake using the
Python language and Python brought the spears back out.
At this point, you should also realize that while Python is amazingly complex and powerful and
very picky about the syntax you use to communicate with it, Python is not intelligent. You are
having a conversation with yourself but using proper syntax.
In a sense when you use a program written by someone else the conversation is between you and
those other programmers with Python acting as an intermediary. Python is a way for the creators
of programs to express how the conversation is supposed to proceed. And in just a few more
chapters, you will be one of those programmers using Python to talk to the users of your
program.
Before we leave our first conversation with the Python interpreter, you should probably know the
proper way to say "good-bye" when interacting with the inhabitants of Planet Python:
>>> good-bye
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'good' is not defined
>>> if you don't mind, I need to leave
File "<stdin>", line 1
if you don't mind, I need to leave
^
SyntaxError: invalid syntax
>>> quit()
You will notice that the error is different for the first two incorrect attempts. The second error is
different because if is a reserved word and Python saw the reserved word and thought we were
trying to say something but got the syntax of the sentence wrong.
The proper way to say "good-bye" to Python is to enter quit() at the interactive chevron >>>
prompt. It would have probably taken you quite a while to guess that one so having a book handy
probably will turn out to be helpful.
1.6 Terminology: interpreter and compiler
Python is a high-level language intended to be relatively straightforward for humans to read and
write and for computers to read and process. Other high-level languages include: Java, C++,
PHP, Ruby, Basic, Perl, JavaScript, and many more. The actual hardware inside the Central
Processing Unit (CPU) does not understand any of these high level languages.
The CPU understands a language we call machine-language. Machine language is very simple
and frankly very tiresome to write because it is represented all in zeros and ones:
01010001110100100101010000001111
11100110000011101010010101101101
...
Machine language seems quite simple on the surface given that there are only zeros and ones, but
its syntax is even more complex and far more intricate than Python. So very few programmers
ever write machine language. Instead we build various translators to allow programmers to write
in high level languages like Python or JavaScript and these translators convert the programs to
machine language for actual execution by the CPU.
Since machine language is tied to the computer hardware, machine language is not portable
across different types of hardware. Programs written in high-level languages can be moved
between different computers by using a different interpreter on the new machine or re-compiling
the code to create a machine language version of the program for the new machine.
These programming language translators fall into two general categories: (1) interpreters and (2)
compilers.
An interpreter reads the source code of the program as written by the programmer, parses the
source code, and interprets the instructions on-the-fly. Python is an interpreter and when we are
running Python interactively, we can type a line of Python (a sentence) and Python processes it
immediately and is ready for us to type another line of Python.
Some of the lines of Python tell Python that you want it to remember some value for later. We
need to pick a name for that value to be remembered and we can use that symbolic name to
retrieve the value later. We use the term variable to refer to the labels we use to refer to this
stored data.
>>>
>>>
6
>>>
>>>
42
>>>
x = 6
print x
y = x * 7
print y
In this example, we ask Python to remember the value six and use the label x so we can retrieve
the value later. We verify that Python has actually remembered the value using print. Then we
ask Python to retrieve x and multiply it by seven and put the newly-computed value in y. Then
we ask Python to print out the value currently in y.
Even though we are typing these commands into Python one line at a time, Python is treating
them as an ordered sequence of statements with later statements able to retrieve data created in
earlier statements. We are writing our first simple paragraph with four sentences in a logical and
meaningful order.
It is the nature of an interpreter to be able to have an interactive conversation as shown above.
A compiler needs to be handed the entire program in a file, and then it runs a process to translate
the high level source code into machine language and then the compiler puts the resulting
machine language into a file for later execution.
If you have a Windows system, often these executable machine language programs have a suffix
of ".exe" or ".dll" which stand for "executable" and "dynamically loadable library" respectively.
In Linux and Macintosh there is no suffix that uniquely marks a file as executable.
If you were to open an executable file in a text editor, it would look completely crazy and be
unreadable:
^?ELF^A^A^A^@^@^@^@^@^@^@^@^@^B^@^C^@^A^@^@^@\xa0\x82
^D^H4^@^@^@\x90^]^@^@^@^@^@^@4^@ ^@^G^@(^@$^@!^@^F^@
^@^@4^@^@^@4\x80^D^H4\x80^D^H\xe0^@^@^@\xe0^@^@^@^E
^@^@^@^D^@^@^@^C^@^@^@^T^A^@^@^T\x81^D^H^T\x81^D^H^S
^@^@^@^S^@^@^@^D^@^@^@^A^@^@^@^A\^D^HQVhT\x83^D^H\xe8
....
It is not easy to read or write machine language so it is nice that we have interpreters and
compilers that allow us to write in a high-level language like Python or C.
Now at this point in our discussion of compilers and interpreters, you should be wondering a bit
about the Python interpreter itself. What language is it written in? Is it written in a compiled
language? When we type "python", what exactly is happening?
The Python interpreter is written in a high level language called "C". You can look at the actual
source code for the Python interpreter by going to www.python.org and working your way to
their source code. So Python is a program itself and it is compiled into machine code and when
you installed Python on your computer (or the vendor installed it), you copied a machine-code
copy of the translated Python program onto your system. In Windows the executable machine
code for Python itself is likely in a file with a name like:
C:\Python27\python.exe
That is more than you really need to know to be a Python programmer, but sometimes it pays to
answer those little nagging questions right at the beginning.
1.7 Writing a program
Typing commands into the Python interpreter is a great way to experiment with Python's
features, but it is not recommended for solving more complex problems.
When we want to write a program, we use a text editor to write the Python instructions into a
file, which is called a script. By convention, Python scripts have names that end with .py.
To execute the script, you have to tell the Python interpreter the name of the file. In a Unix or
Windows command window, you would type python hello.py as follows:
csev$
print
csev$
Hello
csev$
cat hello.py
'Hello world!'
python hello.py
world!
The "csev$" is the operating system prompt, and the "cat hello.py" is showing us that the file
"hello.py" has a one line Python program to print a string.
We call the Python interpreter and tell it to read its source code from the file "hello.py" instead of
prompting us for lines of Python code interactively.
You will notice that there was no need to have quit() at the end of the Python program in the file.
When Python is reading your source code form a file, it knows to stop when it reaches the end of
the file.
1.8 What is a program?
The definition of a program at its most basic is a sequence of Python statements that have been
crafted to do something. Even our simple hello.py script is a program. It is a one-line program
and is not particularly useful, but in the strictest definition, it is a Python program.
It might be easiest to understand what a program is by thinking about a problem that a program
might be built to solve, and then looking at a program that would solve that problem.
Lets say you are doing Social Computing research on Facebook posts and you are interested in
the most frequently used word in a series of posts. You could print out the stream of facebook
posts and pore over the text looking for the most common word, but that would take a long time
and be very mistake prone. You would be smart to write a Python program to handle the task
quickly and accurately so you can spend the weekend doing something fun.
For example look at the following text about a clown and a car. Look at the text and figure out
the most common word and how many times it occurs.
the clown ran after the car and the car ran into the tent
and the tent fell down on the clown and the car
Then imagine that you are doing this task looking at millions of lines of text. Frankly it would be
quicker for you to learn Python and write a Python program to count the words than it would be
to manually scan the words.
The even better news is that I already came up with a simple program to find the most common
word in a text file. I wrote it, tested it, and now I am giving it to you to use so you can save some
time.
name = raw_input('Enter file:')
handle = open(name, 'r')
text = handle.read()
words = text.split()
counts = dict()
for word in words:
counts[word] = counts.get(word,0) + 1
bigcount = None
bigword = None
for word,count in counts.items():
if bigcount is None or count > bigcount:
bigword = word
bigcount = count
print bigword, bigcount
You don't even need to know Python to use this program. You will need to get through Chapter
10 of this book to fully understand the awesome Python techniques that were used to make the
program. You are the end user, you simply use the program and marvel at its cleverness and how
it saved you so much manual effort. You simply type the code into a file called words.py and
run it or you download the source code from http://www.pythonlearn.com/code/ and run it.
This is a good example of how Python and the Python language are acting as an intermediary
between you (the end-user) and me (the programmer). Python is a way for us to exchange useful
instruction sequences (i.e. programs) in a common language that can be used by anyone who
installs Python on their computer. So neither of us are talking to Python, instead we are
communicating with each other through Python.
1.9 The building blocks of programs
In the next few chapters, we will learn more about the vocabulary, sentence structure, paragraph
structure, and story structure of Python. We will learn about the powerful capabilities of Python
and how to compose those capabilities together to create useful programs.
There are some low-level conceptual patterns that we use to construct programs. These
constructs are not just for Python programs, they are part of every programming language from
machine language up to the high-level languages.
input:
Get data from the the "outside world". This might be reading data from a file, or even
some kind of sensor like a microphone or GPS. In our initial programs, our input will
come from the user typing data on the keyboard.
output:
Display the results of the program on a screen or store them in a file or perhaps write
them to a device like a speaker to play music or speak text.
sequential execution:
Perform statements one after another in the order they are encountered in the script.
conditional execution:
Check for certain conditions and execute or skip a sequence of statements.
repeated execution:
Perform some set of statements repeatedly, usually with some variation.
reuse:
Write a set of instructions once and give them a name and then reuse those instructions as
needed throughout your program.
It sounds almost too simple to be true and of course it is never so simple. It is like saying that
walking is simply "putting one foot in front of the other". The "art" of writing a program is
composing and weaving these basic elements together many times over to produce something
that is useful to its users.
The word counting program above directly uses all of these patterns except for one.
1.10 What could possibly go wrong?
As we saw in our earliest conversations with Python, we must communicate very precisely when
we write Python code. The smallest deviation or mistake will cause Python to give up looking at
your program.
Beginning programmers often take the fact that Python leaves no room for errors as evidence
that Python is mean, hateful and cruel. While Python seems to like everyone else, Python knows
them personally and holds a grudge against them. Because of this grudge, Python takes our
perfectly written programs and rejects them as "unfit" just to torment us.
>>> primt 'Hello world!'
File "<stdin>", line 1
primt 'Hello world!'
^
SyntaxError: invalid syntax
>>> primt 'Hello world'
File "<stdin>", line 1
primt 'Hello world'
^
SyntaxError: invalid syntax
>>> I hate you Python!
File "<stdin>", line 1
I hate you Python!
^
SyntaxError: invalid syntax
>>> if you come out of there, I would teach you a lesson
File "<stdin>", line 1
if you come out of there, I would teach you a lesson
^
SyntaxError: invalid syntax
>>>
There is little to be gained by arguing with Python. It is a tool, it has no emotion and it is happy
and ready to serve you whenever you need it. Its error messages sound harsh, but they are just
Python's call for help. It has looked at what you typed, and it simply cannot understand what you
have entered.
Python is much more like a dog, loving you unconditionally, having a few key words that it
understands, looking you with a sweet look on its face (>>>) and waiting for you to say
something it understands. When Python says "SyntaxError: invalid syntax", it is simply wagging
its tail and saying, "You seemed to say something but I just don't understand what you meant, but
please keep talking to me (>>>)."
As your programs become increasingly sophisticated, you will encounter three general types of
errors:
Syntax errors:
These are the first errors you will make and the easiest to fix. A syntax error means that
you have violated the "grammar" rules of Python. Python does its best to point right at
the line and character where it noticed it was confused. The only tricky bit of syntax
errors is that sometimes the mistake that needs fixing is actually earlier in the program
than where Python noticed it was confused. So the line and character that Python
indicates in a syntax error may just be a starting point for your investigation.
Logic errors:
A logic error is when your program has good syntax but there is a mistake in the order of
the statements or perhaps a mistake in how the statements relate to one another. A good
example of a logic error might be, "take a drink from your water bottle, put it in your
backpack, walk to the library, and then put the top back on the bottle."
Semantic errors:
A semantic error is when your description of the steps to take is syntactically perfect and
in the right order, but there is simply a mistake in the program. The program is perfectly
correct but it does not do what you intended for it to do. A simple example would be if
you were giving a person directions to a restaurant and said, "... when you reach the
intersection with the gas station, turn left and go one mile and the restaurant is a red
building on your left.". Your friend is very late and calls you to tell you that they are on a
farm and walking around behind a barn, with no sign of a restaurant. The you say "did
you turn left or right gas station?" and they say, "I followed your directions perfectly, I
have them written down, it says turn left and go one mile at the gas station.". Then you
say, "I am very sorry, because while my instructions were syntactically correct, they
sadly contained a small but undetected semantic error.".
Again in all three types of errors, Python is merely trying its hardest to do exactly what you have
asked.
1.11 The learning journey
As you progress through the rest of the book, don't be afraid if the concepts don't seem to fit
together well the first time. When you were learning to speak, it was not a problem for your first
few years you just made cute gurgling noises. And it was OK if it took six months for you to
move from simple vocabulary to simple sentences and took 5-6 more years to move from
sentences to paragraphs, and a few more years to be able to write an interesting complete short
story on your own.
We want you to learn Python much more rapidly, so we teach it all at the same time over the
next few chapters. But it is like learning a new language that takes time to absorb and understand
before it feels natural. That leads to some confusion as we visit and revisit topics to try to get you
to see the big picture while we are defining the tiny fragments that make up the big picture.
While the book is written linearly and if you are taking a course, it will progress in a linear
fashion, don't hesitate to be very non-linear in how you approach the material. Look forwards
and backwards and read with a light touch. By skimming more advanced material without fully
understanding the details, you can get a better understanding of the "why?" of programming. By
reviewing previous material and even re-doing earlier exercises, you will realize that you
actually learned a lot of material even if the material you are currently staring at seems a bit
impenetrable.
Usually when you are learning your first programming language, there are a few wonderful "AhHah!" moments where you can look up from pounding away at some rock with a hammer and
chisel and step away and see that you are indeed building a beautiful sculpture.
If something seems particularly hard, there is usually no value in staying up all night and staring
at it. Take a break, take a nap, have a snack, explain what you are having a problem with to
someone (or perhaps your dog), and then come back it with fresh eyes. I assure you that once
you learn the programming concepts in the book you will look back and see that it was all really
easy and elegant and it simply took you a bit of time to absorb it.
1.12 Glossary
bug:
An error in a program.
central processing unit:
The heart of any computer. It is what runs the software that we write; also called "CPU"
or "the processor".
compile:
To translate a program written in a high-level language into a low-level language all at
once, in preparation for later execution.
high-level language:
A programming language like Python that is designed to be easy for humans to read and
write.
interactive mode:
A way of using the Python interpreter by typing commands and expressions at the
prompt.
interpret:
To execute a program in a high-level language by translating it one line at a time.
low-level language:
A programming language that is designed to be easy for a computer to execute; also
called "machine code" or "assembly language."
machine code:
The lowest level language for software which is the language that is directly executed by
the central processing unit (CPU).
main memory:
Stores programs and data. Main memory loses its information when the power is turned
off.
parse:
To examine a program and analyze the syntactic structure.
portability:
A property of a program that can run on more than one kind of computer.
print statement:
An instruction that causes the Python interpreter to display a value on the screen.
problem solving:
The process of formulating a problem, finding a solution, and expressing the solution.
program:
A set of instructions that specifies a computation.
prompt:
When a program displays a message and pauses for the user to type some input to the
program.
secondary memory:
Stores programs and data and retains its information even when the power is turned off.
Generally slower than main memory. Examples of secondary memory include disk drives
and flash memory in USB sticks.
semantics:
The meaning of a program.
semantic error:
An error in a program that makes it do something other than what the programmer
intended.
source code:
A program in a high-level language.
1.13 Exercises
Exercise 1 What is the function of the secondary memory in a computer?
a) Execute all of the computation and logic of the program
b) Retrieve web pages over the Internet
c) Store information for the long term - even beyond a power cycle
d) Take input from the user
Exercise 2 What is a program?
Exercise 3 What is is the difference between a compiler and an interpreter?
Exercise 4 Which of the following contains "machine code"?
a) The Python interpreter
b) The keyboard
c) Python source file
d) A word processing document
Exercise 5 What is wrong with the following code:
>>> primt 'Hello world!'
File "<stdin>", line 1
primt 'Hello world!'
^
SyntaxError: invalid syntax
>>>
Exercise 6 Where in the computer is a variable such as "X" stored after the following Python
line finishes?
x = 123
a) Central processing unit
b) Main Memory
c) Secondary Memory
d) Input Devices
e) Output Devices
Exercise 7 What will the following program print out:
x = 43
x = x + 1
print x
a) 43
b) 44
c) x + 1
d) Error because x = x + 1 is not possible mathematically
Exercise 8 Explain each of the following using an example of a human capability: (1) Central
processing unit, (2) Main Memory, (3) Secondary Memory, (4) Input Device, and (5) Output
Device. For example, "What is the human equivalent to a Central Processing Unit"?
Exercise 9 How do you fix a "Syntax Error"?
1
http://xkcd.com/231/